I'm just about to build an aviary it will be built the same as one I have already a breeze block base built on 3x2 slabs sides and front frame of 3x2 timber 10x10mm mesh approx and the roof built with uni strut frame and covered with the plastic sheets found on coservitaries mesh on floor was from 2"x2" hole fence wire cut into removable sized sections for easy removal for cleaning . I don't think your pallet idea is good it will be a night mare to clean out. Good luck.